The progress of any creative discipline changes significantly with the quality of the tools available. As the diversity of user interfaces multiplies in the shift away from personal desktop computing, yesterday's tools and concepts are insufficient to serve the designers of tomorrow's interfaces. My research in human-computer interaction focuses on the earliest stages in UI creation - activities that take a novel idea and transform it into a concrete, interactive artifact that can be experienced, tested, and compared against other ideas. In this talk I will give an overview of different prototyping tools I have built with collaborators to address two research questions: How can tools enable a wider range of designers to create functional prototypes of ubiquitous computing interfaces? And how can design tools support the larger process of learning from these prototypes?